Pacemakers and ICDs carefully monitor characteristics of the heart such as the heart rate to detect arrhythmias, discriminate among different types of arrhythmias, identify appropriate therapy, and determine when to administer the therapy. The device tracks the heart rate by examining electrical signals that result in the contraction and expansion of the chambers of the heart. The contraction of atrial muscle tissue is triggered by the electrical depolarization of the atria, which is manifest as a P-wave in a surface electrocardiogram (ECG) and as a rapid deflection (intrinsic deflection) in an intracardiac electrogram (IEGM). The contraction of ventricular muscle tissue is triggered by the depolarization of the ventricles, which is manifest on the surface ECG by an R-wave (also referred to as the “QRS complex”) and as a large rapid deflection (intrinsic deflection) within the IEGM. The electrical activation detected by the pacemaker on either the atrial or ventricular channel is the intrinsic deflection arising from that specific chamber. Repolarization of the ventricles is manifest as a T-wave in the surface ECG and a corresponding deflection in the IEGM. A similar depolarization of the atrial tissue usually does not result in a detectable signal within either the surface ECG or the IEGM because it coincides with and is obscured by the R-wave. Note that, strictly speaking, the terms P-wave, R-wave and T-wave typically refer only to features of the surface ECG. Herein, however, for the sake of brevity and generality, the terms will be used to also refer to the corresponding signals as sensed internally. Also, where an electrical signal is generated in one chamber but sensed in another, it is referred to herein, where needed, as a “far-field” signal. Hence, a P-wave sensed in the ventricles is referred to as a far-field P-wave. An R-wave sensed in the atria is a far-field R-wave (FFRW).
The sequence of electrical events that represent P-waves, followed by R-waves (or QRS complexes), followed by T-waves can be detected within IEGM signals sensed using pacing leads implanted inside the heart. To help prevent misidentification of electrical events and to more accurately detect the heart rate, the stimulation device employs one or more refractory periods and blanking periods. Within a refractory period, the device does not process electrical signals during a predetermined interval of time—either for all device functions (an absolute refractory period) or for selected device functions (a relative refractory period). As an example of a refractory period, upon detection of an R-wave on a ventricular sensing channel (or upon delivery of a V-pulse to the ventricles), a Post-Ventricular Atrial Refractory Period (PVARP) is initiated on an atrial sensing channel. A first portion of the PVARP comprises a post ventricular atrial blanking (PVAB) interval wherein the pacemaker can detect signals on the atrial channel but does not use the signals for any purpose. The PVAB is provided to prevent the device from erroneously responding to a far-field R-wave on the atrial channel. The PVARP concludes with a relative refractory period during which the pacemaker continues to ignore all signals detected on the atrial channel as far as the triggering or inhibiting of pacing functions is concerned, but not for other functions, such as detecting rapid atrial rates or recording diagnostic information. A total atrial refractory period (TARP) is defined as the period of time including an atrioventricular AV delay, any AV delay extension and the PVARP. The sum of the AV delay and the PVARP define the fastest atrial rate that can be detected to still trigger a ventricular output in a 1:1 relationship.
Accurate detection of heart rates is required, for example, for the purposes of enabling an AMS system wherein the pacemaker switches from a tracking mode such as a DDD mode to a non-tracking mode such as VDI or DDI mode. More specifically, the pacemaker compares a current atrial rate with an atrial tachycardia detection threshold (ATDR) and, if it exceeds the threshold, atrial tachycardia is assumed and the pacemaker switches from the tracking mode to the non-tracking mode. Details regarding AMS may be found in the following patents: U.S. Pat. Nos. 5,441,523 and 5,591,214, which are incorporated herein by reference. See also Levine et al., “Implementation Of Automatic Mode Switching In Pacesetter's Trilogy DR+ And Affinity DR Pulse Generators”, Herzschr. Elektrophys. 10 (1999) 5, S46–S57. Note that DDD, VDI, VVI and DDI are standard device codes that identify the mode of operation of the device. DDD indicates a device that senses and paces in both the atria and the ventricles and is capable of both triggering and inhibiting functions based upon events sensed in the atria and the ventricles. VVI indicates that the device is capable of pacing and sensing only in the ventricles but is only capable of inhibiting the functions based upon events sensed in the ventricles. VDI is identical to VVI except that it is also capable of sensing intrinsic atrial activity. DDI is identical to DDD except that the device is only capable of inhibiting functions based upon sensed events, rather than triggering functions. As such, the DDI mode is a non-tracking mode precluding it from triggering ventricular outputs in response to sensed atrial events. Numerous other device modes of operation are possible, each represented by standard abbreviations of this type.
Thus, an AMS system recognizes when the patient is in an atrial tachycardia such as atrial fibrillation (AF) and switches from the tracking mode to the non-tracking mode to prevent the device from attempting to track the high atrial rates associated with AF. The aforementioned TARP normally prevents the recognition of very rapid atrial rates. However, to facilitate recognition of high atrial rates, the pacemaker can be configured to detect atrial events that coincide with the relative refractory portion of the PVARP. To reduce the likelihood of switching from a tracking mode to a non-tracking mode based on isolated atrial premature beats or a nonsustained run of supraventricular tachycardia (SVT), the AMS system preferably utilizes an averaging technique referred to as a filtered atrial rate interval (FARI) in which all atrial events are counted, including both sensed and paced atrial events, whether captured or not. Filtered atrial rate techniques are discussed in U.S. Pat. No. 5,549,649 to Florio, et al., entitled “Programmable Pacemaker Including an Atrial Rate Filter for Deriving a Filtered Atrial Rate Used for Switching Pacing Modes” and in U.S. Pat. No. 6,128,533 also to Florio, et al., entitled “Pacemaker With Automatic PVARP Adjustment During Automatic Mode Switching”, which are both incorporated by reference herein.
Whereas AMS provides a technique for recognizing an atrial tachycardia should one arise, other techniques have been developed for preventing an atrial tachycardia from occurring. One such technique, referred to as dynamic atrial overdrive (DAO) pacing, is described in U.S. Pat. No. 6,519,493 to Florio et al., entitled “Methods And Apparatus For Overdrive Pacing Heart Tissue Using An Implantable Cardiac Stimulation Device”. With DAO, the pacing device artificially paces the atria at an overdrive rate set to be slightly faster than the intrinsic atrial rate of the patient. It is believed that overdrive pacing helps prevent the onset of atrial tachycardia in part by reducing the number of ectopic beats, i.e. atrial heart beats that do not arise from normal sinus pulses. The DAO pacing rate is controlled to remain generally uniform with the rate is adjusted upwardly or downwardly in response to native sinus or atrial ectopic beats in an effort to prevent the occurrence of atrial fibrillation and other atrial tachyarrhythmias. For the purpose of this discussion, atrial tachycardias and atrial fibrillation shall be used synonymously and interchangeably and refer to any pathologic atrial tachycardia or atrial rate that exceeds the programmed atrial tachycardia detection rate (ATDR). Capture of overdrive pulses may be verified as set forth in U.S. patent application Ser. No. 10/138,438, of Bradley et al., entitled “Method And Apparatus For Providing Atrial AutoCapture In A Dynamic Atrial Overdrive Pacing System For Use In An Implantable Cardiac Stimulation Device”, filed May 2, 2002. The aforementioned patent and patent application are incorporated herein as well.
Thus, DAO provides a technique for preventing the onset of an atrial tachycardia and, should one nevertheless arise, AMS provides a technique for switching to a non-tracking mode so that the high atrial rate is not tracked. DAO is preferably active at all times while the pacemaker is in the tracking mode and is deactivated in the non-tracking mode. Although DAO and AMS are both effective tools, certain problems arise when implemented together. In particular, the following situation may arise. A nonsustained salvo of SVT or multiple atrial ectopic beats can cause the DAO system to increase the atrial pacing rate. On the cycle before release of the next atrial output pulse, an atrial premature beat coincides with the PVARP sufficiently early that it does not conduct to the ventricle. This P-wave is detected by the microprocessor and used to decrement the FARI value, i.e. the detected P-wave causes the rate defined by the FARI to be incremented, which shortens the actual FARI since rate and interval have an inverse relationship. However, since the P-wave coincides with the PVARP, it does not alter the timing interval and thus does not delay release of the next atrial pulse (A-pulse). If the atrial pulse is delivered at a time when the atrial myocardium is physiologically refractory, it will be ineffective. Then, if at the end of the AV delay, a ventricular output (V-pulse) is delivered and captured, the ventricular output can initiate a retrograde P-wave (PR-wave). With the PVARP programmed appropriately to prevent development of a pacemaker mediated tachycardia (PMT), the retrograde P-wave is then not used to trigger a ventricular pulse but is instead used to further adjust the FARI value. The subsequent atrial pulse is again ineffective.
This results in a rhythm termed “repetitive nonreentrant ventriculo-atrial synchronous rhythm” (RNRVAS), which has been described in detail in a paper by Barold and Levine, Journal of Interventional Cardiac Electrophysiology 2001; 5: 45–58 and also described in U.S. patent application Ser. No. 09/795,265 entitled “Implantable Cardiac Device Providing Repetitive Non-Reentrant Ventriculoatrial Synchronous (RNRVAS) Rhythm Therapy Using VA Interval Extension And Method”, of Levine et al., filed Aug. 29, 2002, which is incorporated by reference herein.
In addition, note that a true PMT means that there is a real atrial depolarization that is detected during the atrial alert period. Though retrograde, it is still present. Hence, it will inhibit release of an atrial output pulse even if it were to increase the potential atrial paced rate in accord with DAO. If a Maximum Sensor Rate is set higher than a Maximum Tracking Rate (MTR), then DAO can cause atrial pacing at a rate higher than the MTR and again precipitate a RNRVAS rhythm. RNRVAS rhythm can result in adverse symptoms such as a significant decrease in both blood pressure and cardiac output, palpitations, dizziness and lightheadedness and hence should be avoided.
As a result of RNRVAS, a series of atrial events can arise wherein the pacemaker interprets the rhythm as a tachycardia and initiates mode switching. A—A and PR—PR intervals may each occur at a normal rate (where the A—A interval is the interval between consecutive A-pulses and the PR—PR interval is the interval between consecutive PR-waves.) The pacemaker, as noted, utilizes all usable atrial events (paced and sensed) in the calculation of the FARI value. Hence, as far as the pacemaker is concerned, the actual atrial rates are a combination of the A(ineffective)–P(retrograde) and the P(retrograde)–A(ineffective) intervals. The effect can double the actual rate. Then, when the atrial rate calculated based on the FARI exceeds the ATDR, an inappropriate mode switch occurs and the system exits DAO.
Accordingly, it would be desirable to provide improved FARI techniques for avoiding inappropriate mode switching, particularly as a result of RNRVAS, and aspects of the invention are directed to that end. Note that the techniques described in U.S. patent application Ser. No. 09/795,265 are directed to preventing or detecting the onset and terminating RNRVAS. Techniques of the present invention are instead directed to preventing inappropriate mode switching should RNRVAS occur and may be employed in circumstances wherein RNRVAS detection and termination techniques are not effective or may be employed in devices not configured to provide for RNRVAS detection.
Periods of rapid atrial pacing can also occur in the setting of normal rate modulated behavior even in the absence of DAO. Then, if a premature ventricular complex (PVC) occurs and conducts retrograde or an early atrial premature complex (APC) occurs coinciding with the PVARP and not conducting in an anterograde direction (from atrium to the ventricle through the normal conduction pathway) and if retrograde conduction is intact, an RNRVAS rhythm may occur. As with DAO, the sustained Pr (ineffective)-A-output interval and subsequent A(ineffective)-Pr interval may trigger inappropriate AMS by falsely shortening the FARI causing the detected “atrial” rate to exceed the ATDR. This invention will address this setting as well and will be applicable to devices where DAO is either not available or had not been enabled.
Thus, RNRVAS can result in a situation wherein an atrial tachycardia is detected and a mode switch occurs when, in fact, no true atrial tachycardia is actually present. Circumstances can also arise wherein a true atrial tachycardia has occurred but remains undetected. For example, during a true atrial tachycardia, the amplitudes of the P-waves are sometimes too low to be detected based on the currently programmed atrial sensitivity, and so the atrial tachycardia remains undetected. Accordingly, it would also be desirable to provide improved techniques for detecting the onset of an atrial tachycardia to ensure proper mode switching and further aspects of the invention are directed to that end. In addition, as noted, circumstances can arise wherein a mode switch to the non-tracking mode is performed even though a true atrial tachycardia has not occurred. Accordingly, it would also be desirable to provide improved techniques for verifying that a true atrial tachycardia has occurred and still further aspects of the invention are directed to that end. By providing the foregoing, improved specificity of AMS is thereby achieved.